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Sputnik virophage

http://en.wikipedia.org/wiki/Sputnik_virophage

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Sputnik virophage

Virus classification

Group: Group I (dsDNA)

Order: Unassigned

Family: Unassigned

Genus: Unassigned

Species: Sputnik Virophage

The Sputnik virophage (from "virus" and Greek φάγειν phagein "to eat") has a functional similarity with a bacteriophage. It is an icosahedral subviral agent that is 50 nanometres in size.[1] Sputnik has been found to multiply inside of an Amoeba, although the conditions for this are rather unusual. The Subviral agent is unable to multiply itself inside of the host cell on its own, but when the host cell has been infected sputnik harnesses the viral proteins to rapidly produce new copies of itself. Sputnik has a circular double stranded DNA genome which contains genes able to infect all three domains of life: Eukarya, Archaea and Bacteria. It is associated with the mamavirus, which presumably is related to Acanthamoeba polyphaga mimivirus (APMV). The mimivirus is a giant in the viral world; it has more genes than many bacteria and performs functions that normally occur only in cellular organisms. The mamavirus is even larger than the mimivirus, but the two are very similar in that they form large viral factories and complex viral particles.[2] Virophage growth is deleterious to APMV and results in the production of abortive forms and abnormal capsid assembly of APMV. In one of the experiments done by inoculating A.polyphaga with water containing an original strain of APMV, it was discovered that several capsid layers accumulate unsymmetrically on one side of the viral particle causing the virus to become ineffective. Sputnik decreased the yield of infective viral particle by 70% and also reduced the amoeba lysis by threefold at 24h.[1]

Of the twenty-one predicted protein-coding genes, three are apparently derived from APMV itself, one is a homologue of an archaeal virus, and four others are homologues of proteins in bacteriophages and eukaryotic viruses. Thirteen are ORFans, that is they do not have any detectable homologues in current sequence databases. The Sputnik has a high A + T content (73%) similar to that of APMV.

Several other homologues such as those of a primase–helicase, a packaging ATPase, an insertion sequence transposase DNA-binding subunit, and a Zn-ribbon protein, were detected in the Global Ocean Survey environmental data set, suggesting that virophages could be a currently unknown family of viruses. Considering its functional analogy with bacteriophages, this virus is classified as a virophage (ie a virus that infects other viruses).[3]

Sputnik was found to contain genes that were shared by the mimivirus. These genes could have been acquired by Sputnik after the association of APMV with the host and then interaction between the virophage and the viral host. Recombination within the viral factory might have resulted in the exchange of genes. Sputnik is one of the most convincing pieces of evidence for gene mixing and matching between viruses.

The presence of these genes homologous to the mimivirus in Sputnik suggests that gene transfer between Sputnik and the mimivirus can occur during the infection of Acanthamoeba. Therefore, it is hypothesized that the virophage could be a source of vehicle mediating lateral gene transfer between giant viruses, which constitute a significant part of the DNA virus population in the marine environments. Moreover, the presence of three APMV genes in Sputnik implies that gene transfer between a virophage and a giant virus is crucial to viral evolution.[4]